This invention relates to an .alpha.-Al.sub.2 O.sub.3 -based colored sintered corundum containing metal oxides, to a process for its production and to its use.
The production of abrasion-resistant colored layers is of considerable importance in the ceramic industry and also in other branches of industry.
By virtue of their heat resistance, pigments commonly used for ceramics or enamels are oxides, pure and mixed phases of spinel structure, rutile mixed phases, ZrO.sub.2 mixed phases, zirconium silicate mixed phases and others. These inorganic pigments or rather colored pigments are either colored individual compounds or hosts in which coloring ions are incorporated with mixed phase formation (Ullmann's Encyclopedia of Technical Chemistry, 4th Edition, Vol. 18, pages 599-628).
The low hardness of the pigments imposes limits on the production of highly abrasion-resistant colored layers of the type required, for example, for floor tiles, tools or other parts exposed to heavy wear. The hardness of the pigments is between 5 and 8 on the Mohs scale.
By contrast, corundum is eminently suitable for protection against wear by virtue of its extreme hardness (9 on the Mohs scale). In addition, the high temperature stability and chemical resistance of corundum both have positive effects in this regard. However, since pure corundum is colorless, it is unsuitable for the production of wear-resistant layers which, apart from their protective effect, are also intended to have a decorative character.
Although colored fused corundums containing Cr.sub.2 O.sub.3, NiO, CoO, Fe.sub.2 O.sub.3, TiO.sub.2 and/or V.sub.2 O.sub.5 or other components have been described, the use of these colored fused corundums for the production of colored wear-resistant decorative layers has the disadvantage that only a very limited color spectrum with colors of low intensity is available. Another disadvantage lies in the unfavorable light-refracting behavior of fused corundum so that finely ground corundum appears very light and almost colorless.
In addition, many of the limited range of oxides which can actually be used in the corundum melt on account of its stability are toxic, so that processing in an open arc furnace is extremely problematical.
Most of the known colored fused corundums are unsuitable for decorative purposes.
Layers in which white corundum has been processed together with pigments are attended by the disadvantage that homogeneous layers can only be formed with very considerable effort.
In addition, the pigment present in addition to the corundum undergoes distinctly higher wear than the corundum and is eliminated from the layer under the severe stressing which the corundum is designed to withstand. As a result, there is initially a reduction in the intensity of color. In addition, foreign particles, such as dirt, can collect in the holes formed so that the original color fades to grey.
Overall, abrasion resistance suffers and protection against wear is unsatisfactory.
The problem addressed by the present invention, and the object thereof, are to provide a colored corundum which does not have any of the disadvantages described above.